The most prevalent oral mucosal lesions are aphthous ulcerations commonly referred to as canker sores. The clinical characteristic of oral recurrent aphthous ulceration/stomatitis(ORAS) is well defined and can be partly described as an oval or rounded ulcer covered by a grey-white or yellowish fibrinous exudate and surrounded by an erythematous halo. There is intense or moderate pain and the ulcers heal in about 10 - 14 days for the more common type and more than 2 weeks for the severe type. Recurrence of the ulcers occurs at intervals within a year or over several years. Variations of ORAS described above have made studies on aetiology and treatment difficult to interpret due to differing descriptions of differing diseases with similar clinical signs and symptoms and possibly differing aetiologies. A classification that was considered useful as a working model for ORAS was formulated in 1978. While the classification of ORAS had been widely accepted since 1978, the cause for ORAS is still unknown and its aetiology in general remains unclear. However, its immunopathogenesis is now becoming more clearly defined.